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coolnes 08-20-2007 01:16 PM

basement insulation and vapour barrier
I've read through some of the other threads and would like to share and get some feedback on what I plan for my basement reno.

First some background info:

It's an older home built in 1947.

The basement was previously done 25 or more years ago.
I've ripped out 75% of what was done before.

There used to be rigid insulation from the top of concrete wall to the exterior grade level only.
Then a 2x4 stud wall was built away from that and then insulated with fibre glass R-20 insulation.
Then poly vapour barrier and then drywall or OSB.

Since the fibre glass insulation was touching the concrete walls and there was a vapour barrier, moisture was trapped behind the walls and black mold was growing on the walls below the rigid insulation. Anything that was covered by the rigid did not get mold.

I've read on the building science website that basements should be designed to dry to the interior and that the cold concrete walls should be insulated to prevent the warm moist air from condensing on it. I think what I uncovered proves this point since I can see where the old rigid used to be.

My plan is to use 1.5 inches of rigid insulation glued directly onto the concrete walls with the seams taped.

Then build a 2x4 wall slightly away from the rigid and then just drywall over top of that.
This way, the basement walls are kept warm and nothing can condense on it and any moisture that does get behind the walls aren't trapped there by a vapor barrier. I plan on running a dehumidifier to ensure the humidity doesn't get too high.

Has anyone ever done this?

Also, now that the walls are exposed, I've noticed a couple of small cracks on the walls. They are max 1mm wide.
Any suggestions for filling the cracks?

Also, based on the year of construction, chances are they did not put a damppoofing coating on the exterior.
Should I be doing some kind of damp proofing on the interior or is that pointless since some moisture will find its way in no matter what I do?


SecretSquirrel 08-21-2007 06:59 AM

You may be able to find some useful information at the Dow Website:

Here is a little footnote from one of their online documents.


Building and/or construction practices unrelated to insulation could greatly affect moisture and the potential for mold formation. No material supplier including Dow can give assurance that mold will not develop in any specific system

coolnes 08-21-2007 08:21 AM

I suppose mold can grow if some moisture got behind the insulation.

But the insulation wouldn't trap the moisture there like a poly vapour barrier would. So there's a better chance for the walls to dry out before mold can grow provided that the humidity is low enough.

SecretSquirrel 08-21-2007 08:40 AM

It's a Catch22 actually. Condensation occurs when a cool surface meets warm moist air. A vapor barrier prevents that from occuring. If the masonry wall has the proper waterproofing then in theory, condensation shouldn't occur. Not sure what your situation is but if the masonry is wicking in moisture then I don't know which is the better solution other than eliminating the source of the problem.

Try taping a 2' square piece of poly to the bare wall and see if any condensation occurs behind the poly. Well... you probably already know what's going to happen.

coolnes 08-21-2007 09:49 AM

The basement wall is actually a concrete wall not a masonry wall.

If the vapour barrier was installed perfectly you're correct.

I know the basement gets damp.

I think a better test would be to tape a 1' piece of poly and a 1' piece of rigid insulation and see what happens after a few days.

KUIPORNG 08-21-2007 12:34 PM

in my case, I used roof tar paper then R20 insulation then vapour barrier then drywall... it works well so far... no sign of mold although been up only for less than a year... but I think the tar paper serve as a media to trap any moisture and let it dry gradually should there any minor moisture... rather moisture go into the insulation which is messy I can imagine...

Dannun66 12-01-2007 02:21 PM

I completed a similiar retro on some of my basement rooms and aparently I missed one crucial issue. I applied the rigid insulation directly to the cinder black wall then taped seems and vapor barriered over that then drywalled. I am now getting water on the vinyl flooring. I gather this means I am trapping water inside the wall and it is flowing out the bottom of the wall onto the floor.

Should I have left an air gap and built out that wall first?


Chris Johnson 12-01-2007 03:09 PM


You have actually double vapor barriered. Rigid insulation works as both an air and vapor barrier. I am not questioning you quality work here but nothing is 100% and damp moist air is getting through the littlest of areas on the rigid and now getting trapped by the poly you installed causing it to build up and run down the wall. At some point the studs can become moldy because of the lack of circulating air leading to bigger issues. At some point that poly needs to come out.

Going your route I would have installed 'Z' bar to the concrete wall, filled in between with rigid foam and applied drywall directly to it without any airspace.

concretemasonry 12-01-2007 05:01 PM

I would discourage anyone from using fiberglass in a basement wall. Fiberglass does not absorb moisture, but it holds it very well because of the "dead air" spaces than can never dry out. Fiberglass does not dry out in place! - It takes the movement of DRY air through it.

Everyone looks at a basement wall remodeling through a microscope and is mislead grosly by the R-value concept of just looking at one location. Few basements are finished and insualted 100%, so there are always gaps and/or areas that have different conditions than the "microscopic view" that the pink panther shows.

If you you finish a part of your basement and not the rest, them some people say you should insulate and apply vapor barriers on the interior walls separating different areas with different conditions. - Makes sense!

Every basement is different with different conditions, but rigid insualtion applied to 100% of the basement walls and protected appropriately depending on the use use is the only way to begin finishing. After that, you can build your walls just the way you would do anywhere in the house.

Just a note - Most basements get over-insulated because of advertising and the fact people forget that below grade, the temperature differential between conditioned space and the area outside the insulation is much different below grade than above grade and can reduce the insulation for basements by 80% compared to above grade walls. - For the Canadians - there is really no "magic line" at grade because heat is trandferred equally in all directions in the wall and not just through wall wall.

coolnes 12-05-2007 10:45 AM

I agree that fibreglass insulation should never be used in the basement.
Also, a poly vapour barrier shouldn't be used on the inside. The wall should be allowed to dry to the inside.
Since it's winter now, the humidity is only 30% in my basement. But during the spring and summer, it can get to between 60 and 70%. At that point, I'll be using a de-humidifier as an extra pre-caution to ensure no condensation on the concrete walls. Even though I'm careful to cover everything with rigid insulation, it can never be 100%.

cr8zymike 12-16-2007 11:39 PM


I'm new here and was reading this post.
Quick question for ConcreteMasonary.

I have heard and seen some shows on tv showing rigid insulation and then studding and drywalling over that. I believe this is the method you're referring to. But here's a question. In the basement, i'm thinking of a home office and a rec room. i don't want to have noise from the rec room into the home office. Would I put something like Owens Corning QuietZone acousitc batt in front of the rigid insulation and then drywall that?


CooperChloe 02-01-2009 08:56 AM

basement remodel???
what if you can't insulate all of your basement? I can't get to the area behind the furnace. Wouldn't that defeat the purpose? My plan was to use drylock to seal the concrete, put up metal furring strips (to save room and I have a free set), put a vapor barrier up, and then drywall. I'm hearing from experience homebuilders I know that would be great, but from DIY sites that I have to insulate, etc. I just want to know if what I plan to do will work for years to come. Thanks

p.s. I live in the KC area, so that gives a climate consideration.

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